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New owner of used P12, found a strange delay feedback issue (bug?)

New owner of used P12, found a strange delay feedback issue (bug?)
« on: September 09, 2019, 03:01:02 PM »
Hello, just bought a used P12 on Reverb, have lusted after one of these for a while and finally took the plunge. So far I absolutely love it.

But I ran into an interesting issue that I could not find the answer to online, regarding the delay feedback behavior.

It seems that delay feedback does not act per delay line, but feeds back into all delay lines. For example, if you take a short percussive patch and use delay line 1 to do a 150ms delay, no feedback, and pan that to the left, you can hear a single tap in your left channel, as expected. If you then add delay line 2 with a 250 ms delay and pan it right (no feedback), you can hear a tap in the left channel followed by a tap in the right channel, as expected.

However, adding feedback to either delay line 1 or 2 seems to send the delayed signal through both delay lines, rather than just the one you have applied feedback to. Adding feedback on either tap individually causes the other taps to feed back, and depending on which tap the feedback is added to, sometimes the stereo field collapses or there are taps coming at odd times. Adding feedback to both taps just causes the feedback to kind of come right down the center, no stereo spread.

(This has nothing to do with delay pan BTW, panning just makes it easier to hear what's going on.)

What I would expect to happen is that adding feedback to a single delay line only feeds back that delay line, and adding feedback to a hard panned delay line would only result in more repeats of that delay, panned the same. But apparently it runs through all active delays or something, which seems odd and leads to some unexpected results.

Is this a known behavior? Hopefully I am explaining it well enough, it's simple to repro but kind of hard to describe until you hear it or play around with it.

I am using OS 1.4.0 btw. I didn't see anything in the release notes for the new OS related to this, so I didn't update, but I can do that if it'd help.

Either it's a bug, something is broken on my board, or I just don't understand how it works. Would be nice to know which it is ;D

Thanks,
Dave

Edit: Adding feedback to ONLY delay line 2 (the 250ms delay) in the above example results in the proper ping pong behavior, where both taps repeat alternately left and right. However adding feedback to only #1 doesn't work, and does strange things. Why adding feedback to only #2 results in the taps from #1 to repeat is where I'm confused, but at least I can get the effect I'm after one way or another.
« Last Edit: September 09, 2019, 03:07:48 PM by Savagery242 »

Re: New owner of used P12, found a strange delay feedback issue (bug?)
« Reply #1 on: September 10, 2019, 05:59:21 AM »
 I'm not in front of my P12 at the moment, but that does sound like odd behavior.
 I would for sure get the most recent OS installed. In the latest OS there are 2 feedback modes.
 If the newest OS doesnt fix the issue, try switching feedback mode (it's in a sub menu within each delay tap menu)
 Let us know how it goes. I'll test the behavior on mine in the meantime as well.
 Goodluck!

Re: New owner of used P12, found a strange delay feedback issue (bug?)
« Reply #2 on: September 12, 2019, 09:11:47 AM »
I just updated to the latest 1.4.2.3 and that didn't change anything - setting feedback mode to Low does not change this behavior either.

If it's hard or confusing to reproduce, I can make a short video if that helps. Thanks!

cbmd

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Re: New owner of used P12, found a strange delay feedback issue (bug?)
« Reply #3 on: September 12, 2019, 01:00:00 PM »
The four delays share a memory buffer.  If excess feedback is applied, the buffer can become overrun with a DC signal resulting in the delays seemingly not working.  Setting the feedback mode to LOW will slow the potential for an overrun.

To ease the potential of an overrun in normal and low feedback modes, be conscious of the number of delays you are using and their respective feedback levels.  Turning down the feedback should keep the buffer from overrunning.

I hope this clears things up!
Dave Smith Instruments

cbmd

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Re: New owner of used P12, found a strange delay feedback issue (bug?)
« Reply #4 on: September 12, 2019, 01:01:06 PM »
Also, since the delays share a buffer, any feedback sent back to the buffer is shared by all delays.
Dave Smith Instruments

Re: New owner of used P12, found a strange delay feedback issue (bug?)
« Reply #5 on: September 12, 2019, 02:51:24 PM »
Also, since the delays share a buffer, any feedback sent back to the buffer is shared by all delays.

Thanks for the response! That's interesting, and the shared buffer may explain my problem.

To clarify, I am experiencing that the feedback from one tap is interacting with other taps. So you can't feed back any tap independently. If I feedback tap #1 only, its feedback seems to run through the other taps.

Does that sound right? I am working on a video to show this actually, it's super easy to repro but kind of hard to explain.

Re: New owner of used P12, found a strange delay feedback issue (bug?)
« Reply #6 on: September 13, 2019, 07:46:39 PM »
Ok, I made a video demonstrating exactly what I'm seeing & hearing. Apologies in advance for the amateurish video, but I think it does the trick :D

https://youtu.be/sk8e7fF7FTg

Tonda

Re: New owner of used P12, found a strange delay feedback issue (bug?)
« Reply #7 on: September 14, 2019, 05:33:08 AM »
Because I seldom use the feedback option on the delays I've never experienced this issue. However, after reading this topic I did the same experiment and indeed, the feedback cannot be used independently on any delay tap. But, it is not that unexpected now I'm thinking about it. Most delay units that I'm aware of, with 2 or more delay taps, share the feedback buffer. As cbmd explained, the P12 doesn't behave differently, all taps share the same buffer. But the fact is that the P12 has a feedback knob for every delay tap separately. The question is: why has every delay tap a separate feedback option while they share the same buffer? Maybe it would be better if there was just one global feedback knob?
« Last Edit: September 14, 2019, 05:59:29 AM by Tonda »